In August, three Bahraini human rights groups accused the Gulf Arab monarchy's NSA of the systematic use of torture.

A security official said at the time it would investigate the allegations.

In 2011, Bahrain put down an uprising by pro-democracy activists, many of them Shiites. The monarchy believes the opposition seeks to overthrow it by force and accuses Iran of aiding in deadly militant attacks on security forces.

Home to the US Fifth Fleet, Bahrain denies opposition claims that it marginalises Shiites economically and in government representation.